A satellite positioning system (SPS), such as the Global Positioning System (GPS), typically comprises a system of earth orbiting satellite vehicles (SVs) enabling user equipment (UE) such as cellular telephones, personal communication system (PCS) devices, and other mobile stations (MSs) to determine their location on the earth based, at least in part, on signals received from the SVs.
An SPS user can derive precise navigation information including three-dimensional position, velocity and time of day through information gained from the SVs. Reception of signals from four SVs enables precise position determination in four dimensions (latitude, longitude, altitude, and time). However, one disadvantage of a particular SPS system for location determination is the relatively long time needed to perform signal acquisition under certain conditions. SV signals cannot be acquired until they have first been located by searching in a two-dimensional search “space,” whose dimensions are code-phase delay and observed Doppler frequency shift. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case after a receiver “cold start,” a large number of code delays and frequencies may be searched for each SV signal that is to be acquired and tracked. These locations are examined sequentially, a process that can take several minutes in a conventional SPS receiver.
A SPS receiver acquires signals from SVs whenever the receiver has lost reception, such as, after power down, or when the signal has been blocked from the receiver for some period of time. Assuming a fixed sensitivity threshold, the time spent acquiring the SV signals is typically proportional to a total search space derived from a product of time and frequency uncertainty. The signal re-acquisition delay may take tens of seconds if the search space is large.
In order to reduce this delay, information may be provided to aid an SPS receiver in acquiring a particular signal. The goal of such SPS assistance information is to permit a wireless mobile station (MS) to predict the time of arrival, or code phase, of a particular SV signal, and the Doppler shift of the SV signal. If the MS is provided with an initial reference position that is within an area of predefined size, such as a particular cellular coverage, then the total search space can be reduced to that consistent with the predefined size.
However, assisted position location systems depend upon communication with an external entity. Such communication suffers from connection and messaging latency, consumes additional power and consumes additional communication system bandwidth that impacts the overall capacity.